The present study of unintentional, intentional, and undetermined drowning death showed a mean incidence of 3.1/100, 000 and an average overall decrease of 2% each year. In Sweden, the average annual number of drowning deaths during the study period was half of traffic deaths . Notably, Australian researchers estimate that compared to traffic deaths the risk of drowning is 200 times higher when calculating the person-time exposures, indicating that drowning may merit interest from a prevention perspective .
The mean incidence of unintentional drowning deaths calculated from our Swedish death data was 1.6/100,000 inhabitants. In other Nordic countries, the corresponding figures range from 1.4/100,000 in Denmark to 6.1/100,000 inhabitants in Finland [5, 21]. For 2005, the incidence was comparable with studies from Australia and the USA (1.3 and 1.2/100,000, respectively) [20, 22].
Differences in incidence of drowning in various countries could be due to differences in demography, geography (e.g., presence of coast, lakes, and other water sources), the implementation of preventive measures (e.g., swimming skills, use of floating devices such as a life vest, using lifeguards to monitor public beaches, and fencing around pools), and risk taking behaviour [1, 23]. Furthermore, different ways of collecting data and coding practices may also lead to differences in statistics [5, 24]. From an international perspective, the incidence of drowning deaths among children was very low in the present study. However, older age groups still have a markedly high incidence of drowning, a finding also reported in previous studies from other countries [5, 13]. This finding may be explained by more effective prevention for younger groups than for older age groups . Especially in high-income countries several preventive measures have successfully been implemented for children and have led to a decrease in drowning deaths and hospitalization after drowning incidents .
There was a male predominance in unintentional drowning, a finding that previous studies also have noted [2, 13, 21, 22, 26, 27]. The explanations for this might be that males participate in more water activities and are also likely to take more risks, especially after drinking alcohol . In the present study, most unintentional drowning deaths occurred in lakes and at sea, but the most frequent location for females was a bathtub. The water location and activities of unintentional drowning cases vary according to access to sea and other water sources in different regions and countries [5, 13, 21]. It should be noted that there are approximately 520,000 lakes in Sweden . The regional differences in Sweden especially regarding the incidence of unintentional drowning deaths could partly be explained by differences in access to water sources, leisure activities, and the length of ice coverage during winter, which in northern Sweden could last up to five months.
Drowning is a common method of suicide [9, 29–32] and in the present study suicide constitutes 31% of all cases and 7% of all suicides in Sweden. Notably, in Sweden suicidal drowning is most frequent in females especially in the age groups 50 to 79 years. A similar result has been presented in a study from Croatia . Suicide is a sign of psychiatric illness, which constitutes an important public health problem . As other studies have noted [35, 36], the suicide rate increases with age. The combination of depression and availability to bathtubs and other bodies of water may explain the high incidence of suicidal drowning. In the present study, psychoactive drugs were present in 71% of suicides, indicating that many of those individuals may have had contact with health care. The manner of suicide differs between males and females ; males tend to commit suicide in more violent ways and females tend to commit suicide by drowning or by overdosing on drugs [31, 37]. Suicidal drowning and previous suicide attempts have a strong correlation, so it is particularly important to identify individuals suffering from depression and mental illnesses and initiate effective care and treatment to avoid suicides . Health professionals dealing with suicidal cases need to be aware of the risk (especially for women) of drowning as a method to commit suicide often in combination with pharmaceutical drugs and alcohol.
Homicide due to drowning was uncommon in our study. However, in one-third of the homicides children (0-16 years of age) were the victims. The detailed circumstances could not be revealed in this study, but it has been reported that parents may kill their children by drowning them .
A substantial proportion (44%) of unintentional drowning deaths tested positive for alcohol in the blood. This finding could be compared with 51% in Finland , 35%-55% in the USA , 50% in New Zealand , 62% in Ireland , and 22% in Australia . The proportion of alcohol in drowning deaths may reflect the differences in alcohol policy and consumption in these countries.
In our study, 54% of boating fatalities were alcohol positive, a percentage that is comparable with a study from the USA . The relative risk of dying in boating incidents increases with higher blood alcohol level and is increased 16-fold at a BAC of 1.0 g/l . Legislation may affect the use of alcohol when driving a boat. In Sweden, until 2010 the legal limit for alcohol in drivers of larger boats (greater than ten metres) was 0.5 g/l; later this limit was lowered to 0.2 g/l, which is the same as for drivers of motor vehicles in road traffic. At present, there is no limit for alcohol in drivers of smaller recreational boats (less than ten metres) although it seems obvious this should be implemented.
For road traffic, it is well known that alcohol impairs a person’s judgment, performance, and behaviour [43, 44] and increasing levels of impairment are associated with increased BAC levels . Moreover, alcohol affects coordination and increases the risk of hypothermia . Therefore, reducing alcohol use in combination with water activities may prevent many of these deaths .
In the present study, alcohol was found in 24% of suicidal drowning and alcohol may also have facilitated suicide. It has been reported that alcohol lowers the threshold for suicide; the presence of alcohol was 23% for suicidal drowning death compared to 38-64% for more violent suicide methods . Consequently, heavy drinkers with a history of mental illness might be at higher risk of suicide [35, 48].
Pharmaceuticals and illicit drugs
Almost 25% of those who drowned had one or more pharmaceuticals in their blood. Benzodiazepines and antidepressants were most common, especially in the older age groups. Psychoactive drugs affect cognitive function, concentration, vision, coordination, and balance depending on brain concentration, individual susceptibility, and interaction with other drugs . These drugs may affect risk-taking behaviour as well as affect survival possibilities after falling in the water, i.e., swimming ability. For boating, psychoactive drugs may play a similar role as for drivers in road traffic where benzodiazepine and antidepressants have been correlated with increased crash risk [50, 51]. Those who committed suicide had the highest percentage of psychoactive drugs, which probably is related to a higher psychiatric morbidity. The combination of alcohol and psychoactive drugs might have an additional effect on cognitive functions . However, it is difficult to know how individuals are affected as this depends on interaction with other drugs and whether higher dosages than therapeutic recommendations were used. In this study, illicit drugs were detected in almost 10% of the cases and it was higher in the undetermined group. However, most of the drowning deaths were not tested for illicit drugs. More extensive testing is needed to obtain more reliable data to assess the role of illicit drugs in drowning deaths.
The findings in this study suggest that it is important to reach middle/older age groups when designing future prevention programs, a recommendation also suggested by American and Australian researchers [13, 53]. Education, knowledge, and training are important factors. The information about the danger of combining alcohol and drugs with water activities should be given to groups at high risk, i.e., males and the middle/older age groups. Eventually, it has previously been found that it is more difficult to protect adults than children from drowning . Prevention strategies for adults should include a focus on swimming ability, ice prods, and personal floatation devices that could affect survival (i.e., the time in water). Immediate rescue and cardiopulmonary resuscitation by bystanders are also important factors that affect survival . It is also important to emphasise passive measures such as designing bridges in such a way that persons cannot reach the edge, making it more difficult to jump or fall from a bridge.
Strengths and limitations of the present study
To our knowledge, this is the first comprehensive study of drowning in Sweden. As the autopsy rate is high in Sweden for drowning deaths, probably only a few cases were missed. All autopsied cases of drowning in Sweden were analysed, including unintentional, intentional, and undetermined drowning. This approach provides a complete data set. As this type of thoroughness is often not the case in other studies, it is difficult to compare the figures with previous studies. In our study, we have information about alcohol, pharmaceutical drugs, and illicit drugs in femoral blood, whereas many previous studies on drowning death do not provide data on drugs .
Of all the drowning deaths, 18% of the cases were undetermined. According to a previous study that analysed the coding practices of injury deaths in Nordic countries, Sweden had a higher rate of undetermined cases and many of these were water related . Therefore, the coding practice might underestimate drowning suicides in Sweden.
The data on alcohol in drowning deaths should be used with caution. Post-mortem microbial activity can produce misleading alcohol levels . To avoid overestimation of alcohol, we have excluded decomposed bodies from alcohol analyses and the cut-off level for alcohol was set to 0.2 g/l in all cases. Therefore, our data represent a conservative estimate. Another way to limit this problem is to perform blood sampling within 24 hours after immersion , but this was not possible in all cases. It is difficult to compare studies since information about the proportion of decomposed bodies and source when obtaining the blood sample for alcohol measurement and toxicology screening often are missing. In the present study, most of the drowning deaths were not tested for illicit drugs, so there might be a selection bias.
Because the present study was based on register data, information regarding some circumstances could not be obtained, i.e., pre-drowning activities, swimming skills, use of personal floatation devices, and rescue attempts.